Tween-based microemulsions: a percolation view

Abstract

Results obtained on electrical conductivity, density, ultrasonic velocity and viscosity for the macroscopically homogeneous, stable, optically transparent and isotropic media — generally called microemulsions, are presented for a number of systems incorporating non-ionic surfactant: Tween 20, water, benzene and alkanol (C 1–C 8,C 10) as a function of volume fraction of water ( φ) at 30°C. The experimental path followed in each system has been characterized by a fixed molar ratio of chosen alkanol to the surfactant ( n a/ n S)=2. Water is used as titrating component hence varying the value of ω from 0 to 180 within the system, where ω is the molar ratio of the aqueous phase to Tween 20+alkanol. In this paper, a phenomenological theory for the effects of interactions on the conductivity of water in oil microemulsions in the dilute limit of spherical droplets is analyzed. The density ( ρ m) and the isentropic compressibility ( k s,m) of the micellar phase are derived from the experimental density ( ρ) and ultrasonic velocity ( u) data. The results indicate a trend towards an enhanced water like character of the dispersed phase at high volume fraction of water ( φ). Viscosity varies in a non-monotonic way, giving two peaked plots. In the phase diagram, the realms-of-existence of single phase (i.e. microemulsion formation), double phase and mesophase have been delineated for the system containing propanol as co-surfactant. A simple structural model has been applied for the calculation of the various parameters i.e. aggregation number ( n), core radius ( r n ) and surface number density of the surfactant molecules at the interface ( α S).